I'd say Walmart needs a high-load solution, as does paypal and LinkedIn who are both using node. Static vs dynamic languages both have merits and are almost completely subjective, but I'd say it comes down to team quality. Don't use the sharp tools if you have undiciplined or inexperienced devs who will fail to design correctly and write tests. Walmart has Eran Hammer, so I don't think they need to worry about that.

You can write Node.js backends in TypeScript with static typing and refactoring support.

Yeah, when I saw they released the Hapi[1] framework for Node several months ago my initial reaction was "is Walmart Labs part of THE Walmart??"

1. http://spumko.github.io/ (Hapi used to be maintained within the Walmart Labs Github account but now falls under Spumko)